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The adhesive properties, as measured by bulk tack analysis, are found to decrease in blends of isomerically pure Sc3N@Ih-C80 metallic nitride fullerene (MNF) and polystyrene-block-polyisoprene-block-polystyrene (SIS) copolymer pressure sensitive adhesive (PSA) under white light irradiation in air. Reduction of tack is attributed to the in-situ generation of 1O2 and subsequent photooxidative crosslinking of the adhesive film. Comparisons are drawn to classical fullerenes C60 and C70 for this process. This work represents the first demonstration of 1O2 generating ability in the general class of metallic nitride fullerenes, (M3N@C80). Additional support is provided for the sensitizing ability of Sc3N@Ih-C80 through the successful photooxygenation of 2-methyl-2-butene to its allylic hydroperoxides in benzene-d6 under irradiation at 420 nm, a process which occurs at a comparable rate to C60. Photooxygenation of 2-methyl-2-butene is found to be influenced by the fullerene sensitizer concentration and oxygen gas flow rate. Molar extinction coefficients are reported for Sc3N@Ih-C80 at 420 nm and 536 nm. Evaluation of the potential antimicrobial activity of films prepared in this study stemming from the in-situ generation of 1O2 led to an observed 1 log kill for select Gram-positive and Gram-negative bacteria.
The photophysical properties of empty-cage fullerenes (e.g., C60, C70), their ability to generate singlet oxygen, and potential impact on medical applications have been well-documented.1–14 This phenomenon is not limited to classical empty-cage structures, as several other types of fullerenes have emerged recently with the capacity to sensitize the formation of singlet oxygen. In 2001, members of the azafullerene fullerene family, e.g., (C59N)2 and C59NH, joined classical empty-cage fullerenes in reports of singlet oxygen formation,15,16 but with about half the efficiency of C60. In the same year, several classical endohedral metallofullerenes (EMFs), e.g., Dy@C82 and Gd@C82, were also shown to generate singlet oxygen and efficiently sensitize the photooxidation of olefins in an ene reaction.17 Often this ene reaction is used as a diagnostic tool for the generation of singlet oxygen, where the detection of photooxygenated products is attributed to the singlet oxygen mediated process.18–21 EMFs have generated a great deal of excitement over their empty-cage cousins, as the inclusion of a metal or metals within the fullerene cage offers increased potential resulting from the unique properties of the encapsulated metal. Possible applications under development include their use in optoelectronic devices and energy conversion systems.17 Metallic nitride fullerenes, MNFs, e.g. Sc3N@C80 and Gd3N@C80, represent an even more recent addition to the metallofullerene class of compounds, and are characterized by the incorporation of a metal-nitride complex, M3N, inside an all carbon C80 cage. Although, there have been no literature reports of singlet oxygen generation from this class of compounds, other application areas which are discussed include their use as MRI contrast agents22,23 and in photovoltaic devices.24
A consequence of the ability of C60 to generate singlet-oxygen is the development of new application areas such as fullerene-based, antimicrobial agents. Kai et al.8 have demonstrated the antibacterial activity of C60 dissolved with poly(vinylpyrrolidone) K30. Fang et al.25 have shown the effect of C60 on both Gram-negative and Gram-positive bacteria in terms of their phospholipid composition. Lyon et al26–28 have described the antibacterial activity of fullerene water suspensions, and Li et al.29 provide an overview of antimicrobial nanomaterials such as C60 for water disinfection and microbial control. One of the central technical efforts of our fullerene-polymer research has been the study of how to incorporate metallofullerene nanomaterials into macrostructures (e.g. dendrimers, polymers, films, devices) and exploit their benefits as multi-functional, and in many cases, stimuli-responsive materials, for example, MRI active + antimicrobial.
Polymer blends and crosslinked networks containing fullerene nanomaterials may have potential for use in photovoltaic cells,30–32 non-linear optical devices,33–36 and as oxygen sensors.37 Our group’s research interests lie in discovering the unique properties of polymer-nanocomposite structures containing fullerene-based nanomaterials. In this effort, we recently reported the chemical, mechanical, and adhesive properties of interesting stimuli-responsive nanocomposites prepared from rubber-based, pressure sensitive adhesive (PSA) – C60 fullerene blends.38,39 PSAs are widely utilized in commercial tape, label manufacturing, and medical adhesives, and may be produced to have a variety of chemical compositions, such as acrylic, silicone, and rubber-based.40 Rubber-based systems are typically highly flexible and elastic, and the polystyrene-block-polyisoprene-block-polystyrene (SIS) copolymer is an example of a commonly applied thermoplastic elastomer-based PSA.41 These block copolymers have high cohesive strength due to the physical crosslinking, which results from the microphase separation of the hard, high Tg, polystyrene segments from the rubbery portions derived from the unsaturated polyisoprene center blocks.42
Our group has demonstrated that the adhesive properties of rubber-based elastomeric adhesives, such as SIS, can be dramatically affected when blended with common photochemical sensitizers for the formation of singlet oxygen, including rose bengal, acridine, and C60 fullerene.38,39 The photochemical efficiency of a photosensitizer to generate singlet oxygen is the singlet oxygen quantum yield, ΨΔ, and is reported to be high (0.8 – 1) for acridine, rose bengal, and C60 fullerene in common solvents at visible wavelengths.3–7,21 Solubility variations, knowledge of singlet oxygen quantum yields, and wavelength dependent absorption extinction coefficients as solid film constituents limited detailed direct comparisons; however, C60 fullerene was consistently the superior singlet oxygen generator as measured by a subsequent loss in adhesion of our nanocomposite systems. Purge gas experiments confirmed that the presence of oxygen was essential to the mechanism of adhesive loss, and in combination with the effects of added singlet oxygen generators and scavengers, these results supported a singlet-oxygen mediated process.
As part of our continuing interest in discovering the unique properties of polymer-nanocomposites containing fullerene-based nanomaterials, we have extended this photochemical study to include isomerically purified Sc3N@Ih-C80, Figure 1, and evaluated its ability to produce singlet oxygen in solution and polymer films. This behavior, when extended to other MNF species, has the potential to produce a unique family of stimuli-responsive materials containing functional metals. These MNF-polymer nanocomposite films may hold promise for photovoltaic, photonic, electronic, and biomedical applications. With regard to specific applications, the films produced in this study were evaluated for potential antimicrobial activity resulting from the in-situ generation of singlet oxygen.
Triblock polymer SIS (D1161) was provided by Kraton Polymers, Inc. (Belpre, Ohio) and was comprised of ~15 wt % polystyrene stabilized with 0.14 wt % Irganox 565 antioxidant. GPC analysis yielded Mw fractions of 315,800 (64 %) and 102,500 (36 %) and polydispersities of 1.08 and 2.45. Toluene (>99.9 % HPLC grade), d6 benzene (>99 %) and 2-methyl-2-butene (>99.5 %) were purchased from Sigma Aldrich. C60 and C70 were purchased from MER Corporation (Tucson, AZ). The production, separation, and isolation of isomerically pure (> 99 %) Sc3N@Ih-C80 used in this study was performed by our new, non-chromatographic purification method as previously described.43,44 Unless otherwise indicated, all materials were used as received without further purification.
Solutions for photochemical studies were prepared by adding a catalytic amount of the fullerene sensitizer, C60 or Sc3N@Ih-C80, to benzene-d6 solvent with brief sonication to dissolve solids. The 2-methyl-2-butene was added to the sensitizer solution, which was then transferred to a 250 mL, borosilicate glass jacketed photochemical reaction vessel (Ace Glass Inc.). Irradiation was conducted using a Rayonet photochemical reactor fitted with seven, 420 nm λmax bulbs. Reaction temperature was maintained at 15 °C by use of a circulating chiller, which was connected to the jacketed portion of the reaction vessel. The system was purged at a constant rate with dry O2 for 30 minutes prior to initiating exposure and maintained for the duration of the reaction. Aliquots of approximately 0.7 mL were removed from the reaction vessel via syringe and analyzed immediately using a 300 MHz/52 MM Bruker NMR. Peroxide products were reduced in a 1M triphenyl phosphine solution for safe handling and disposal. Molar extinction coefficients were determined for fullerene sensitizers through Beer’s Law plots of a serially diluted 7.75 × 10−5 M C60 or Sc3N@Ih-C80 starting solution followed by UV/VIS analysis on a Shimadzu UV-2401PC Spectrometer.
SIS polymer was dissolved in a toluene solution containing C60 or C70 fullerenes or Sc3N@Ih-C80 MNF and stirred overnight in the dark to prevent early exposure to light. Using an eight-path wet film applicator (Paul N. Gardner Company, Inc.), bulk tack samples were drawn on Q-panel brand test panels from 20 wt % solid solutions in toluene, followed by solvent evaporation in a dark hood overnight. The prepared films averaged 25–30 μm thickness and were visually uniform. Unless otherwise indicated, film samples were irradiated using a 150 W tungsten/halogen white light source. The radiation intensity was measured at the sample (30 s @ 22 °C) with a “power puck” photometer to give 0.004 W/cm2 visible, and no measurable UV-A, UV-B, or UV-C.
Bulk tack studies were conducted on the TA. XTplus Texture Analyser (Godelming, Surrey, UK). An applied force (35 g) on the one inch round probe tip (57R stainless steel) and a probe insertion speed of 0.1 mm/s gave an insertion depth of 10 % film thickness. The applied force was held for 10 s, and then the probe tip was withdrawn at a constant rate of 0.1 mm/s. The applied mass required to remove the probe tip from the film was obtained in grams per unit time, and the highest point was recorded as the peak tack. The probe tip was cleaned with toluene solvent and dried after each tack experiment.
Sensitizer-containing polymer solutions were prepared as described above and flow coated onto glass microscope slides which were maintained in the dark until the bacterial challenge was conducted. These slides were prepared in triplicate at 0.2 and 1.0 wt % fullerene sensitizer. Control slides were prepared by similarly flow coating SIS polymer solution containing no fullerene sensitizer.
Luria-Bertani (LB) media (Difco Laboratories, Detroit, MI) was used as a bacterial growth medium for preparation of bacteria for bacterial challenges and was prepared according to manufacturer’s specifications. Staphylococcus aureus (ATCC 25923) was used for all Gram-positive bacterial challenges, and Escherichia coli (ATCC 25922) was used for all Gram-negative bacterial challenges.
Overnight cultures were grown in LB media, pelleted, and resuspended in 0.5 % saline solution at a concentration of 109 CFU/mL. An aliquot of 20 uL (107 CFU) of the bacterial suspension was deposited as a liquid droplet using a calibrated pipette onto a 1 cm2 area of each coating. The bacteria were exposed to white light under ambient conditions for 2 h. The coatings were then swabbed with a sterile cotton swab that had been dipped in LB broth to ensure bacteria recovery and then placed in 5 mL of LB broth. The broth was then serially diluted seven times. The dilution series were incubated at 37 ºC for 18 h, after which tubes were visually examined and determined to have growth by the presence of turbidity. Log kill was determined by the following relationship: Log kill = 7 – highest dilution exhibiting bacterial growth.
Our group has found that the bulk tack measurement is capable of not only monitoring the gain or loss in adhesive properties, but also provides mechanistic insight into the chemical species produced during irradiation. For example, as more polar species are produced, such as peroxides, an increase in tack is observed. Using FTIR analysis, peak tacks can be correlated to the production of these reactive peroxide intermediates. A preferred molecular structure of a PSA could be generally described as a network having minimal crosslink density and a sufficiently low plateau modulus to yield a high compliance of the adhesive and good contact with the surface. Extensive chemical crosslinking in the elastic segment results in the increase of the modulus above the Dahlquist’s criterion ~105 Pa. The photochemically altered polymer adhesive films therefore exhibit more glassy behavior, and tack is reduced.
Figure 2 illustrates the changes in the adhesive bulk tack as a function of fullerene sensitizer identity and visible exposure time in fullerene-SIS composite adhesive films. Bulk tack of control films, prepared without sensitizer, remains relatively unchanged as a function of exposure time, thus negating a significant thermal contribution to the tack measurement. The temperature of test coupons increases approximately 20 °C over the first 30 min of irradiation, using a 150 W tungsten/halogen lamp at 6″ distance from samples to source, and remains constant thereafter. Incorporation of as little as 2.0 × 10−4 mmol sensitizer/gram of polymer leads to dramatic effects on the tack/time adhesive plot. This low concentration of sensitizer – C60, C70, and Sc3N@Ih-C80 – was employed to better discern any differences among the fullerene series and provide relative kinetic details. C60 and C70 fullerenes possess similar sensitizing ability, when incorporated into SIS films, and the tack/time plot shows the now characteristic parabolic shape just prior to the complete loss of tack. This transient increase in tack has been attributed to the generation and subsequent decomposition of reactive peroxide intermediates during irradiation. When incorporated into SIS adhesive films, isomerically pure Sc3N@Ih-C80 does lead to a loss in adhesive tack, however, at a reduced rate relative to C60 and C70 classical fullerenes. The shape of the Sc3N@Ih-C80 tack/time plot is also characteristic of the singlet oxygen mediated process. Knowledge of singlet oxygen quantum yields and wavelength dependent absorption extinction coefficients as solid film constituents are required for direct comparisons; however one can conclude that under these conditions, Sc3N@Ih-C80 is less able to productively generate singlet oxygen as measured by polymer-singlet oxygen chemical reactions leading to the loss of adhesive tack.
To provide further support for the ability of Sc3N@Ih-C80 to photochemically generate singlet oxygen, solution studies were performed in a photochemical reactor under visible irradiation, using a modified procedure from that reported by Shinohara.17 In this process, evidence for singlet oxygen generation is detected by the successful photooxygenation of 2-methyl-2-butene to the allylic hydroperoxides shown, Scheme 1. As was observed by Shinohara, this reaction produces a ~1:1 ratio of the two hydroperoxide products 1 and 2. Each species possesses a well-resolved and unique hydrogen chemical shift (Ha, 4.93δ; Hb, 3.97 δ; and Hc, 5.50 δ) in the 1H NMR, corresponding to the area of one hydrogen and leading to the ready analysis of reaction mixtures using integration of 1H NMR peak areas. The reaction time was optimized at 1 hour to yield ~10 % conversion of starting material and thereby avoiding the appearance of side products resulting from the degradation of peroxide products.
Table 1 summarizes the solution photochemistry of Sc3N@Ih-C80 relative to C60 fullerene for the photooxidation of 2-methyl-2-butene via in-situ generated singlet oxygen. All reactions are conducted at a 0.3 M concentration of 2-methyl-2-butene using a catalytic amount of fullerene photosensitizer. In the absence of fullerene sensitizer, no detectable products were obtained. However, a significant conversion to hydroperoxide products was observed in the presence of either C60 or Sc3N@Ih-C80, and given the experimental error associated with our process, little difference between the two types of fullerene sensitizers could be detected. Reducing the photocatalyst by an order of magnitude resulted in a significant decrease in the production of hydroperoxide products. A similar effect was observed by reducing the oxygen purge rate. This is not surprising since the production of singlet-oxygen generated products is expected to depend on the partial pressure of oxygen in the reaction environment.45
The molar extinction coefficients were determined in toluene solvent for isomerically purified Sc3N@Ih-C80 at 420 and 536 nm and are reported in Figure 3, along with C60 values for comparison. The wavelength of 420 nm is chosen to coincide with our photochemical lamp emission λmax. The 536 nm is an approximate lambda max of the C60 visible spectrum. Beer’s law plots of prepared dilution series produced linear plots, and good correlation values.
Several classical (non-MNF) endohedral metallofullerenes have been investigated as both singlet oxygen generators by Shinohara17 and as singlet oxygen quenchers by Yanagi.46 An example energy diagram, which describes the interaction between excited state C60 fullerene and ground state molecular oxygen, can be approximated by Figure 4. C60 is efficiently converted to the triplet excited state (3S*) upon UV irradiation, which efficiently sensitizes the formation of singlet oxygen (1O2) through an energy transfer mechanism. Once generated, singlet oxygen is free to engage in additional molecular reactions. Shinohara found that Dy2@C2n, Dy@C82, Gd@C82 all successfully generated high yields of photooxidation products of 2-methyl-2-butene, while La@C82 produced no reaction.17 This work was followed recently by Yanagi, who determined the total quenching rate constant (sum of physical and chemical quenching mechanisms) for La@C82 and several other fullerenes, and found that La@C82 possessed a quenching rate constant comparable to β-carotene.46 This process was discussed in terms of a combination of energy and charge transfer processes. The triplet state of the metallofullerene should lie slightly higher in energy than the 1O2 energy for an efficient energy transfer mechanism to occur. Our preliminary findings have significant data to support the generation of singlet oxygen from Sc3N@Ih-C80, tentatively placing a lower limit on the excited state energy of Sc3N@Ih-C80. A continued investigation to measure more accurate rates and efficiencies for 1O2 generation, probe potential quenching mechanisms and chemical reactivity considerations is ongoing.
Films prepared using C60 and Sc3N@Ih-C80 sensitizers were also evaluated for potential antimicrobial activity stemming from the in-situ generation of 1O2 under white light, and these results are summarized in Table 2. Techniques for biological assays were adapted from previously published reports.47,48 Biological results for control samples – no sensitizer – afforded no biological activity against the two pathogenic bacteria examined in this study. A small additive effect was observed for fullerene-sensitized films, where C60 and Sc3N@Ih-C80 provided a 1–2 log kill of both Gram-(+) and Gram-(−) bacteria. Multiple trials of each system were performed, and we suggest that C60 is a more active antimicrobial agent in these systems than Sc3N@Ih-C80. This finding is consistent with the photochemical film study, where tack measurements showed generation of peroxide intermediates at a later time under identical conditions. Unfunctionalized fullerenes are hydrophobic and therefore insoluble in water, which prevents a direct comparison to solution anti-microbial studies. The ability of fullerenes to generate singlet oxygen is strongly influenced by chemical modification of the cage and other solubilizing procedures. However, a recent article by Markovic et al.5 offers a current review of the photosensitization ability of select fullerenes, such as C60, and their potential application as powerful antimicrobial agents.
In conclusion, we have evaluated the singlet oxygen generating ability of Sc3N@Ih-C80 in solution and SIS adhesive films. This work represents the first demonstration of the singlet oxygen generating ability of this family of compounds and the specific photochemical sensitizing ability of Sc3N@Ih-C80. The rate of singlet oxygen generation leading to oxidative crosslinking and subsequent loss of tack in adhesive films was slower for Sc3N@Ih-C80 than for the classical fullerenes C60 and C70, which had comparable rates. A reduced relative rate of singlet oxygen production from Sc3N@Ih-C80 is further supported by antimicrobial studies. However in solution photochemical studies, where evidence for production of singlet oxygen is supported by the successful photooxygenation of 2-methyl-2-butene to its allylic hydroperoxides, product yields suggested similar sensitization activity for Sc3N@Ih-C80 and C60.
JPP thanks NSF CHE-0847481 and NIH R15AG028408 (National Institute on Aging). Additional support from the Office of Naval Research (JHW) and NSF CHE-0547988 (SS) is also acknowledged. Graduate Student Fellowships for MAM (NSF GRFP) and CEC (Department of Education, GAANN #P200A060323) are also acknowledged.